Lee: Here are a few shot of how I did it. I probably have some better ones, but my home computer is curently down, so I only have the ones I loaded on Photobucket. Anyway, I built a 1X1 seat back frame that has 2 kickbacks into the trunk area for strength, and is also tied into the body with some strapping. My seat back bolts to it, so it can be taken out at a later date if needed. The kickbacks help define the trunk sides and are securely anchored through the plywood floor. There is no flex and no gap at the seat. If you go this route, you will need to figure out the necessary angle to put the framing at as it will set the set back angle.
Wow. Thanks Ben. I have look at your "Indy Album" many times. These under construction photos really help. That trunk seperation is part of what I was looking for. Because of the amount of space I actually have a right hand set of Mazda Miata seat adjusting rails.(release handle will be on drivers left. This is beacause there is no ONE personal owner this car (belongs to Polk County Public School system.) We are also using an early 90 Mustang GT tilt steering column.
Your 4 link rear suspension also served us as inspiration too
Lee